The present invention relates to a process for manufacturing a hydrostatic bearing of porous material, and more specifically to a process for manufacturing hydrostatic bearings of porous material, with less dispersion in performance.
Recently, when the high accuracy is required in machining, a hydrostatic bearing such as an air spindle or air slider is used. The air spindle is excellent in high speed performance and rotation accuracy, whereas the air slider is excellent in high speed performance and guiding accuracy.
An example of the conventional hydrostatic bearings is an orifice restrictor compensated type hydrostatic bearing, whose air feeding hole for compressed air are made by drilling. However, recently, the hydrostatic bearing made of porous material, is becoming popular. When the bearing surface is made of porous material, innumerable orifices of small diameters can be obtained, and therefore hydrostatic bearings of excellent performance can be manufactured relatively easily.
As the porous material used as raw material for such a hydrostatic bearing, brittle material is used in order to prevent plugging (blinding) of the bearing surface, which can occur during finishing process. Typical examples of the brittle material are carbon, graphite and ceramics.
In the process for manufacturing a hydrostatic bearing of porous material, the bearing surface is sealed with resin partially, to make the diameter of the micropores of the surface layer smaller than that of the base material, in order to prevent the pneumatic hammer (self-induced vibration) due to the compression of air in the pores of the porous material, or to optimize the flow rate by reducing the dispersion in grain diameter of porous material and the non-uniformity of the distribution of the micropores.
For example, Jpn. Pat. Appln. KOKAI Publication No. 63-88317 discloses a process as follows. That is, a porous graphite material is machined to form bearing surface, and resin is impregnated into the bearing surface. Then, part of the resin is removed using solvent while monitoring the flow of air permeating from the bearing surface. Thus, the flow of permeating air is adjusted.
Jpn. Pat. Appln. KOKAI Publication No. 2-256915 discloses another process as follows. That is, the surface of a porous material is immersed into a liquid thermosetting resin. The immersion is carried out until the resin impregnates from the surface to a predetermined depth. After that, the impregnated resin is cured by heat, and thus a surface restrictor layer with micropores being sealed partially, is formed on the surface of the porous material.
However, the former process entails a drawback in productivity in that it requires time and skill to impregnate the resin or to get rid of the resin once impregnated, since the flow rate of permeating air is adjusted by controlling the amount of resin impregnated. The latter process entails a drawback in uniformity of performances of products, that it is not always easy to control the depth of the impregnation of the resin to be constant.
The present invention has been achieved as a solution to the drawbacks of the conventional processes for manufacturing hydrostatic bearing of porous material, and the object thereof is to provide a process for manufacturing a hydrostatic bearing, which has a high productivity and less dispersion in performance.
According to the present invention, there is provided a process for manufacturing a hydrostatic bearing of porous material for supporting a movable member with hydrostatic pressure of fluid fed from a bearing surface, the process comprising the steps of:
machining a surface of a base member made of a first porous material;
covering the surface of the base member with a surface restrictor layer made of a second porous material having micropores of a cross sectional area smaller than that of the base member; and
removing a surface portion of the surface restrictor layer by machining, to form bearing surface;
wherein the depth of machining of the surface portion of the surface restrictor layer is controlled such that flow of the fluid fed from the bearing surface is adjusted to a target value.
According to the process of the present invention, the surface restrictor layer serves to control the flow rate of the fluid passing therethrough. Consequently, the thickness of the surface restrictor layer can be controlled more accurately by machining, because the flow of liquid fed from the bearing surface can be accurately adjusted, by measuring the flow rate, to achieve a target value.
In the case where porous material made of non-ferrous metal such as bronze is used as the first porous material for the base member, it is preferable that the surface of the base member should be finished by cutting with single crystal diamond bite. Therefore, the surface of the base member can be finished without plugging (blinding) of the surface.
In the case where porous material made of brittle material such as ceramic is used as the first porous material for the base member, the surface of the base member is finished by grinding or lapping. Therefore, the surface of the base member can be finished without plugging of the surface.
In the case where porous material made of solid lubricant such as molybdenum disulfide or carbon is used as the second porous material for the surface restrictor layer, it is preferable that the surface portion of the surface restrictor layer should be machined by, grinding, lapping or cutting with use of a single crystal diamond bite. Therefore, the surface portion of the surface restrictor layer can be finished without plugging of the surface.
In the case where porous material made of brittle material such as ceramic is used as the second porous material for the surface restrictor layer, it is preferable that the surface portion of the surface restrictor layer should be machined by grinding or lapping. Therefore, the surface portion of the surface restrictor layer can be finished without plugging of the surface.
It is preferable that while the surface portion of the surface restrictor layer is being machined, compressed gas is supplied to the rear side of the surface restrictor layer via the base member, and the flow of the compressed gas fed via the surface restrictor layer is measured. On the basis of the flow rate of the compressed gas thus measured, the depth of machining is controlled. In this manner, it becomes possible to accurately adjust the flow of the fluid fed from the bearing surface.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.